GB2334625A

GB2334625A - Calibrating antenna array

Info

Publication number: GB2334625A
Application number: GB9803670A
Authority: GB
Inventors: Raymond Owen; Nicholas William Anderson; Paul Howard
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1998-02-24
Filing date: 1998-02-24
Publication date: 1999-08-25
Also published as: GB9803670D0

Abstract

The invention provides a technique for calibration of an antenna array system whilst allowing the system to receive a wanted signal. The antenna array system (100) comprises an antenna array (101) containing antenna elements (103) each connected to a receiver chain (105). The output of each receive chain is connected to a combiner (107). The weighted summation of the signals from the different signals allows the gain pattern of the antenna array to be varied. Calibration is performed by inserting a known narrow-band signal at the antenna elements. This signal will be processed together with one or more wanted signals. One or more characteristics (for example the transfer function) of the receive chain is determined from evaluating the known signal after it has been processed in the receive chain and based on this characteristic the antenna system is adjusted. Finally the known signal is removed from the signal in the receive chain thereby permitting normal reception of the desired signal. The known signal may be inserted directly into the receiver chain, or by a test transmitter in the antenna's polar diagram.

Description

METHOD OF CALIBRATION FOR ANTENNA ARRAY SYSTEMS AND ANTENNA ARRAY SYSTEM Field ofthe Invention The invention relates to an antenna array system with calibration and a method therefor.

Badgrund to the Invention Antenna array systems consisting of a plurality of antenna elements and a combiner for combining the signals from each antenna element are known to provide substantial benefits for many radio communication systems.

The gain of an antenna array to signals incident upon it can be made to vary with the angle of arrival of these signals by deriving an output signal from a weighted sum of the individual signals incident on the individual elements. Many signal processing techniques can be utilised to give the desired processing gain in order to extract a wanted signal in the presence of one or more interferers.

Variations in receive chain gain and phase across the branches of the array can seriously degrade the performance of these techniques as they introduce an error term to the desired gain and phase weightings. As such, an accurate calibration of each branch must be performed. This calibration must be continuously updated to overcome both long term and short terms variations, for example arising from temperature variations or ageing of the components in the individual receive chains.

Current techniques require that a calibration signal is injected into the receive chain and detected whilst no traffic is present. This is an obvious disadvantage, and for example for cellular communication systems much of the capacity gain which can be achieved by antenna array systems can be lost due to calibration overheads.

A system for calibrating antenna array systems whilst allowing normal operation of the receiver is thus desired.

Summary of the Invention This invention seeks to provide a method and system for calibration of an antenna array system whilst allowing the system to receive one or more wanted signals simultaneously.

According to the invention, there is provided a method for calibration of an antenna array system with a plurality of antenna elements each connected to a receive chain comprising the steps of: inserting a known signal in at least a first receive chain; processing a combination of said known signal and a received radio signal simultaneously in at least said first receive chain; determining a first characteristic of said first receive chain from said first known signal; adjusting said antenna array system in response to said first characteristic; and attenuating said known signal from said combination of said known signal and said received radio signal.

The known signal may be a narrow band signal such as an unmodulated carrier, and may be inserted directly into the receive chain or may be transmitted from an external antenna.

According to a different aspect of the invention there is provided an antenna array system with a plurality of antenna elements each connected to a receive chain comprising: means for inserting a known signal in at least a first receive chain processing a combination of said known signal and a received radio signal; a processor for determining a first characteristic of said first receive chain from said first known signal; a controller for adjusting said antenna array system in response to said first characteristic; and means for attenuating said known signal from said combination of said known signal and said received radio signal.

Brief Description of the Drawings An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 is an illustration in block diagram form of an antenna array system according to prior art.

FIG. 2 is an illustration of a flow chart for calibration of an antenna array system in accordance with the invention.

FIG. 3 is an illustration of some sample frequency spectra of signals processed by a receive chain of an antenna array system.

FIG. 4 is an illustration in block diagram form of an example of how a known signal can be inserted and monitored in an antenna array system.

FIG. 5 is an illustration in block diagram form of an example of one branch of an antenna array system in accordance with the invention.

FIG. 6 is an illustration of an example of beam forming of an antenna array system according to the invention.

FIG. 7 is an illustration in block diagram form of an example embodiment of the invention.

Detailed Description of a Preferred Embodiment FIG. 1 illustrates a typical antenna array system 100. An antenna array 101 contains a number of antenna element 103 each connected to a receiver chain 105. Each antenna element 103 and receive chain 105 forms a branch of the antenna array system 100. The output of each receive chain is connected to a combiner 107. The receive chain will typically amplify, filter and down convert the received signal to complex baseband as is well known in the art. The receive chain will thus contain conventional filters, amplifiers, mixers and oscillators and variations in all of these will affect the received signal. The combiner 107 performs a weighted addition of the signals from the received signals in a known manner. The weights can be complex valued thereby modifying both the phase and amplitude prior to the addition. Alternatively, the phase rotation and amplitude adjustment can be performed in the receive chains, typically by adjusting the phase of the down conversion oscillators and the gain of the amplifiers. For this alternative, the combiner will simply perform a summation of all the signals. Typically, the receive chain will also contain an analog to digital converter for converting the received analog signal into a digital signal for easier processing.

The weighted summation of the signals from the different branches allows the gain pattern of the antenna array to be varied as is well known in the art. However, the technique relies on accurate adjustment of phases and amplitudes of the received signals of each branch, and are as a consequence very sensitive to variations in the transfer function of the receive chain. Hence, it is essential to calibrate the receive system to compensate for variations between the individual receive chains. The calibration can either compensate for differences by adjusting the individual receive chains or analogously by changing the weightings before the summation.

According to the invention, a method is provided for calibrating an antenna array system whilst allowing the system to receive one or more wanted signals simultaneously. FIG. 2 illustrates a flowchart 200 of the method of the invention. Step 201 consists in inserting a known signal at one or more of the antenna elements. This signal will be processed together with one or more wanted signals in step 203. This step corresponds to a conventional receiver for an antenna array system but in this case it is the combination of the received radio signal and the inserted known signal which is processed. In step 205 one or more characteristics of the receive chain is determined from evaluating the known signal, after it has been processed in the receive chain. For example, the phase response of the receive chain may be determined from evaluating the phase of the known signal before and after it has been processed by the receive chain. Step 207 consist in adjusting the antenna system according to the estimated characteristic. For example, if a phase response of the receive chain is determined, the adjustment will consist in phase rotating the signal of that receive chain by the sign inverse of the estimated phase response, prior to the combining of the signals. Finally, step 209 consist in attenuating the known signal from the combination of the known signal and the received radio signal.

The calibration is thus performed by evaluating how a known signal is modified in the individual receive chain and compensating accordingly, followed by the attenuation of the known signal from the combination of the two signals thereby leaving only the received radio signal.

FIG. 2 illustrates the method by five sequential steps. However, preferably the method is implemented by some or all of the process steps running in parallel which allows for the calibration to be carried out continuously.

Preferably, the method is also applied to all antenna elements and receive chains in parallel. However, each element can be calibrated sequentially, intermittently or the calibration can be limited to only a fraction of the antenna elements. The calibration can furthermore be an absolute calibration of a receive chain compensating for the variations of this receive chain, or it can be a relative calibration of one receive chain with respect to another. For example, a known signal can be inserted simultaneously in two or more branches and the phase response of one receive chain with respect to the other can be determined by comparing the phase difference between the known signal in the two branches.

Preferably, the inserted known signal is a narrow band signal such as an unmodulated carrier. A known signal which is narrow band in comparison to the bandwidth of the desired received radio signal will interfere in a limited frequency band and hence a narrow notch filter centered on the center frequency of the narrow band signal can significantly attenuate the known signal without significantly degrading the desired signal. FIG. 3 illustrates an example where the desired signal has a power spectrum 301 much broader than the power spectrum of the inserted known signal 303, which in this case is a pure unmodulated carrier. A narrow notch filter with a frequency response 305 as shown in FIG. 3 will substantially attenuate the inserted known signal but only have negligible effect on the desired signal. Many data signals transmitted over radio contains no DC component and their power spectra have a low power density at the carrier frequency. Furthermore, many receivers use a high pass baseband filter centered at DC as this removes the degradation due to DC offset. Preferably the inserted known narrow band signal is therefore centered very close to DC of the base band signal. This corresponds to the center frequency of the known signal being very close to the carrier frequency of the desired received radio signal.

The known signal can be fed to the receive chains in different ways. An example is shown in FIG. 4, where only one branch of an antenna array system 400 is shown. The branch contains an antenna element 103 and a receive chain 105 connected to a combiner 107 as previously described. In addition an RF (Radio Frequency) coupler 401 is inserted which adds the received signal and a known signal from a signal source 403. The output of the receive chain is monitored by a processor 405 which compares the processed known signal at the output of the receive chain with the known signal at the output of the signal source 403 in accordance with the invention.

An alternative to directly inserting the known signal in the receive chain is to use an external antenna which radiates a signal that is received by the antenna elements. For an antenna with a known location in respect of the antenna array the phase and gain relationship for the signal received on the individual antenna can be determined. By transmitting a known signal from a known location, a known signal is thus received by the antenna elements and thus inserted in the receive chain. This known signal can then be used for calibration in accordance with the invention.

One alternative for determining a characteristic of the receive chain is to compare the signal at the output of the receive chain with one or more local replicas of the signal. For example, the signal at the output of the receive chain can be correlated with a number of replicas of the known signal each representing a different phase rotation. The phase response of the receive chain can thus be found from the phase rotation of the local replica with the highest correlation. The received radio signal present in the signal at the receive chain output will in this case correspond to noise in the correlation. The correlation will thus have to correlate over a sufficiently long time interval for the correlation to be reliable. The method is mainly applicable to an antenna array system where the signal is sampled in the receive chain, so that the local replicas can be stored and the correlation can be performed digitally.

However, preferably the known signal is separated from the combination of the known signal and the received radio signal before the characteristic of the receive chain is determined. For example, for a known signal being an unmodulated carrier as illustrated in FIG. 3, a very narrow low pass filter will be used to separate the known signal from the received radio signal which has a much wider bandwidth. This substantially reduces the noise introduced in the estimation process by the received radio signal.

Additionally, a local estimate of the known signal on the output of the receive chain can be generated and the known signal can be removed at the receive chain output by subtracting this estimated known signal.

FIG. 5 illustrates one branch 500 of an antenna array system incorporating these techniques. A known signal c(t) is added to the received radio signal r(t) in the coupler 501 or by an external antenna as previously described. The signal Rc(t)= c(t)+r(t) is processed in the receive chain 503 with a transfer function Hrc(t) yielding the signal at the output of the receive chain 503 Rrc(t)=Rc(t)*Hrc(t)=[c(t)+r(t)]*Hrc(t)=c(t)*Hrc(t)+r(t)*Hrc(t) where * denotes convolution.

The known signal is extracted in a filter 505 and one or more parameters of the known signal are estimated in an estimator 507. Preferably, an estimate of the transfer function of the receive chain is generated from these parameters. A stored local replica of the known signal 511 is convolved with the estimated transfer function He(t) in a processor 509 generating an estimate of the known signal at the output of the receive chain. A subtractor 513 subtracts the estimated known signal from the total signal yielding Rr(t)=Rrc(t)-c(t)*He(t)= c(t)*Hrc(t)+r(t)*Hrc(t)- c(t)*He(t)= c(t)* [Hrc(t)-He(t)J + r(t)*Hrc(t) For an accurate estimate of the transfer function of the receive chain, the known signal can thus be removed completely from the total signal.

However, due to the noise and uncertainty in the estimation some residual of the known signal will typically remain in the signal. When estimating the parameters of the extracted known signal the transfer function of the filter 505 must be taken into consideration. However, the filtering and estimation is preferably performed in the digital domain. The filter characteristic will therefore be known and constant and hence can easily be compensated for. Typically, the receive chain will have much larger bandwidth than the narrow band known signal, and the estimation of the receive chain transfer function can be limited to finding a gain and phase rotation of the receive chain.

According to a different aspect of the invention the known signal is inserted in a plurality, preferably all, of the branches of the antenna array system and the signal attenuation of the known signal is achieved by the weighting of the individual branches in the summation. According to this invention the combining of the signals in the individual branches is such that the signal components of the received radio signal add coherently while the signal components of the known signal add destructively. This is identical to the known manner of forming beam and nulls in an antenna array system, and corresponds to placing a beam in the direction of the received radio signal and a null in the direction of the known signal. FIG.

6 illustrates the technique where the desired received radio signal is transmitted from an antenna 605 and received by an antenna array consisting of six antenna elements 601. At the same time a known signal is transmitted from a different antenna 603 in accordance with the invention.

A beam 607 is formed in the direction of the antenna 605 by adding the signals from each antenna elements so that signals from this direction add coherently. At the same time, the weights are chosen so that the signals arriving from the direction of the antenna 603 add destructively corresponding to a null 609 being formed in this direction. In this alternative, the known signal is thus present in all branches for performing the calibration and is removed in the combiner by choosing the weights to form a null in the corresponding direction. The method is equally applicable to the known signal being inserted directly in the receive chains as being transmitted by an external antenna. For example, if an unmodulated carrier is inserted in all branches of a half wavelength spaced antenna array with a relative phase difference of 180 degrees this corresponds to the signal being received from the end-fire direction, i.e. the direction parallel to the antenna array. When combining the signals from the different branches an equal weighting of all signals will cancel the inserted signal completely corresponding to forming a null in the end-fire direction.

In accordance with this alternative, the method of calibration can be implemented so that adjusting the antenna array system is performed on the basis of the known signal component in the combined signal. This corresponds to evaluating a characteristic of the receive chains given as the attenuation achieved in the combining of the signals. An example is illustrated in FIG. 7 where the branches containing the antenna elements 103 and receive chains 105 are connected to the combiner 107 as previously described. The known signal is subsequently extracted from the combined signal in for example a filter 701. The filtered signal is fed to an estimator 703 which determines a characteristic of the known signal component of the combined signal and a controller 705 derives an adjustment according to this. FIG. 7 illustrates the adjustment being carried out in the combiner 107 by modifying the weights of the individual branches.

A specific example of the approach is where the known signal is separated from the combined signal in a low pass filter 701 and the estimator 703 simply determining the level of the known signal. This level is used as an error signal by the controller 705 which drives the weights of the individual branch to minimise this error signal. Alternatively, an error signal may be generated as a ratio between the received radio signal and the known signal in the combined signal, corresponding to the signal to noise ratio of the combined signal.

According to a different aspect of the invention, the known signal is extracted from the combination of the known signal and the received radio signal in each of the branches and all the extracted signals are fed to the same estimator. This estimator determines the direction of arrival in a known manner and compares the estimated direction of arrival of the known signal with the expected direction of arrival. The difference is subsequently used for determining an adjustment to the antenna array system. As a specific example, the difference in the direction of arrival may be used as an error signal in a feedback system controlling the weights of the combiner. Furthermore, the weighting of the individual branches is performed so that a null is formed in the estimated direction of arrival of the known signal, thereby attenuating the known signal in the combined signal from the combiner.

It will almost always be sufficient to limit the adjustment of the antenna array system to a phase and gain compensation for each individual branch. This can be achieved by modifying the weight of that branch accordingly as in the example of FIG. 7. Alternatively, the adjustment of the antenna array system can be performed by directly modifying the receive chain, preferably by adjusting the phase of the down conversion local oscillator and the gain of one or more of the amplifier stages in the receive chain. Any known technique for adjusting the parameters of the antenna array system may be substituted without subtracting from the invention. Likewise, many techniques for estimating a characteristic (e.g. the transfer function) of a receive chain based on a known signal are known, and any of these may be substituted without subtracting from the present invention. As previously described, the attenuation of the known signal can be achieved in the individual branch, in the combiner or by processing the combined signal at the output of the combiner. Any known technique for attenuating a signal can furthermore be used.

Implementation of an antenna array system incorporating the method described will be apparent to the skilled person. Preferably, the implementation of the method will be in a software program running on a suitable processor such as a micro processor or a digital signal processor.

The present invention thus provides a technique for calibration of an antenna array system in which a radio signal can be received simultaneously with the calibration being performed. Hence, no capacity loss is caused by the calibration system.

Claims

Claims 1. A method for calibration of an antenna array system with a plurality of antenna elements each connected to a receive chain comprising the steps of: inserting a known signal in at least a first receive chain; processing a combination of said known signal and a received radio signal simultaneously in said first receive chain; determining a first characteristic of said first receive chain from said known signal; adjusting said antenna array system in response to said first characteristic; and attenuating said known signal from said combination of said known signal and said received radio signal.
2. A method as claimed in claim 1 wherein said known signal is a narrowband signal.
3. A method as claimed in claim 2 wherein said narrowband signal is an unmodulated carrier.
4. A method as claimed in claim 3 wherein a first carrier frequency of said unmodulated carrier is substantially identical to a second carrier frequency of said received radio signal.
5. A method as claimed in claim 1 or 2 wherein said known signal is attenuated by filtering.
6. A method as claimed in claim 1 wherein said known signal is coupled directly to said first receive chain.
7. A method as claimed in claim 1 wherein said known signal is transmitted by an external antenna and received by said plurality of antenna elements.
8. A method as claimed in claim 1 wherein said first characteristic of said first receive chain is determined by comparing a first local replica of said known signal with said combination of said known signal and said received radio signal as processed by said first receive chain.
9. A method as claimed in claim 1 wherein the step of determining a first characteristic of said first receive chain from said known signal comprises the steps of: separating an extracted known signal from said combination of said known signal and said received radio signal; estimating a first estimated characteristic of said extracted known signal; and determining said first characteristic from said first estimated characteristic.
10. A method as claimed in claim 9 wherein the step of separating an extracted known signal comprises filtering of said combination of said known signal and said received radio signal.
11. A method as claimed in claim 9 wherein said first characteristic is an impulse response of said first receive chain estimated from said first estimated characteristic.
12. A method as claimed in claim 9 where an estimated known signal is generated from said first estimated characteristic and the step of attenuating said known signal from said combination of said known signal and said received radio signal is performed by subtracting said estimated known signal from said combination of said known signal and said received radio signal.
13. A method as claimed in claim 9 wherein said first characteristic of said first receive chain is determined by comparing a first local replica of said known signal with said extracted known signal.
14. A method as claimed in claim 1 where said method is applied to a plurality of receive chains.
15. A method as claimed in claim 14 wherein the step of attenuating said known signal is performed by generating a combined signal by combining a plurality of signals from said plurality of receive chains so that said known signal is attenuated from said combination of said known signal and said received radio signal.
16. A method as claimed in claim 15 wherein said first characteristic is a level of said known signal in said combined signal is determined.
17. A method as claimed in claim 16 wherein the step of adjusting said antenna array system in response to said first characteristic is characterised by minimising said level of said known signal.
18. A method as claimed in claim 16 wherein the step of adjusting said antenna array system in response to said first characteristic is characterised by maximising a ratio of a level of said received radio signal and said level of said known signal in said combined signal.
19. A method as claimed in claim 14 wherein said first characteristic is a direction of arrival of said known signal.
20. A method as claimed in claim 19 wherein the step of attenuating said known signal is performed by combining a plurality of signals from said plurality of receive chains so that a null is formed in said direction of arrival.
21. An antenna array system with a plurality of antenna elements each connected to a receive chain comprising: means for inserting a known signal in at least a first receive chain processing a combination of said known signal and a received radio signal; a processor for determining a first characteristic of said first receive chain from said known signal; a controller for adjusting said antenna array system in response to said first characteristic; and means for attenuating said known signal from said combination of said known signal and said received radio signal.
22. An antenna array system as claimed in claim 21 wherein said processor comprises: means for separating an extracted known signal from said combination of said known signal and said received radio signal; an estimator for estimating a first estimated characteristic of said extracted known signal; and means for determining said first characteristic from said first estimated characteristic.
23. An antenna array system as claimed in claim 22 further comprising means for generating an estimated known signal from said first estimated characteristic, and wherein said means for attenuating said known signal from said combination of said known signal and said received radio signal are operable to subtract said estimated known signal from said combination of said known signal and said received radio signal.
24. An antenna array system as claimed in claim 21 wherein said known signal is inserted in a plurality of receive chains and the means for attenuating said known signal are operable to generate a combined signal by combining a plurality of signals from said plurality of receive chains so that said known signal is attenuated from said combination of said known signal and said received radio signal.
25. An antenna array system as claimed in claim 21 wherein said known signal is inserted in a plurality of receive chains and wherein said first characteristic is a direction of arrival of said known signal.
26. An antenna array system as claimed in claim 25 wherein the means for attenuating said known signal are operable to combine a plurality of signals from said plurality of receive chains so that a null is formed in said direction of arrival.
27. A method for calibration of an antenna array system substantially as hereinbefore described with reference to the accompanying drawings.
28. An antenna array system substantially as hereinbefore described with reference to the accompanying drawings.